1. Field of the Invention
The present invention relates to a universal joint. More particularly, the present invention relates to a universal joint constructed in such a manner that when axial pressure is applied to the universal joint in a state in which rotational torque is applied to the universal joint, one or more guide balls fitted in one or more rows of guide holes formed through a circumferential wall of a slip bush come into contact with the inner circumferential surface of the tube, whereby the guide balls reduce sliding frictional force while producing rolling frictional force so that the universal joint can be smoothly collapsed, and that when torsional fracture torque is applied to the slip bush, the guide balls also serve as supports, thereby improving the torsional fracture strength of the slip bush, which in turn improves a safety factor.
2. Description of the Prior Art
FIG. 1 exemplifies a steering apparatus employing a universal joint. In a steering apparatus for a vehicle, a lower end of a steering shaft 130 is connected to a gear box 180, wherein the steering shaft 130 should be connected to the gear box 180 at an inclined angle variable according to circumferential conditions. In order to meet this requirement, a universal joint 120 is employed.
Here, such a universal joint 120 typically includes a tube 100, a shaft 110 and a slip bush 200.
Such a universal joint 120 is connected to the steering shaft 130, which is connected with a steering wheel 170, at one end and is connected to the gear box 180 at the other end, thereby transmitting rotary power produced at the steering wheel 170 to wheels of the vehicle through the gear box 180.
A steering column 150 includes an outer tube 145, an inner tube 140 and a mounting bracket 160, wherein the outer tube 145 is formed at the steering wheel 170 side and the inner tube 140 has a diameter smaller than the inner diameter of the outer tube 145, so that the inner tube 140 can be inserted into the outer tube 145 when an impact is applied. Because each of the outer tube 145 and the inner tube 140 is formed from a hollow tube, the steering shaft 130 can be smoothly rotated.
FIG. 2A is an exploded perspective view of a conventional universal joint. As shown in FIG. 2A, the universal joint, which is connected to the steering shaft 130 so as to transmit the rotary movement of the steering wheel 170 to the gear box 180, includes a tube 100 and a shaft 110, wherein the shaft 110 is introduced into the tube 100 with a slip bush 200 interposed between them.
The tube 100 engaged with the outer circumferential surface of the slip bush 200 has an inner circumferential surface processed in a proper shape for smoothly guiding C-type parts 210 and solid parts 220, which are formed around the circumferential surface of the slip bush 200. The shaft 110 engaged with the inner circumferential surface of the slip bush 200 has an outer circumferential surface, a longitudinal end portion of which is processed to have a shape which is complementary to the shapes of the solid parts 220 and the C-type parts 210 on the inner circumferential surface of the slip bush 200, so that the longitudinal end portion can be introduced into the slip bush 200.
The engagement structure of the tube 100 and the shaft 110 with the slip bush 200 interposed between them is adapted to be extended or retracted in the axial direction thereof so as to make it possible to absorb an impact transmitted from wheels of the vehicle, as well as to improve the assemblability thereof. The engagement structure is adapted to absorb kick-back loads occurring due to impacts applied to the wheels of the vehicle by the unevenness of a road on which the vehicle travels. In addition, the engagement structure is also adapted to facilitate the assembly of the steering column 150 and the gear box 180 by being axially slid when the steering column 150 and the gear box 180 are assembled.
FIG. 2B is a perspective view of the conventional slip bush. As shown in FIG. 2B, the slip bush 200 made from a plastic material has three C-type parts 210 and three solid parts 220. The slip bush 200 is cylindrical, and the C-type parts 210 and the solid parts 220 are alternately formed around the circumferential surface of the slip bush 200 at regular intervals.
Each of the solid parts 220 is formed in a solid cylinder extending in the longitudinal direction of the slip bush 200, wherein a half of each solid part is formed on the outer circumferential surface of the slip bush 200 and the remaining half is formed on the inner circumferential surface, so that the halves are symmetric to each other. Each of the C-type parts 210 is formed in a hollow cylinder extending in the longitudinal direction of the slip bush 200, wherein each C-type part 210 also has two halves symmetrically formed on the inner and outer circumferential surfaces of the slip bush 200, respectively, and the half formed on the outer circumferential surface is slit in the longitudinal direction thereof.
FIG. 2C is a cross-sectional view of the conventional universal joint. As shown in FIG. 2C, the slip bush 200 interposed between the shaft 110 and the tube 100 is formed from a plastic material in a cylindrical shape and has three C-type parts 210 and three solid parts 220 alternately formed around the circumferential surface of the slip bush 200 at regular intervals.
The slip bush 200 has a configuration that makes the solid parts 220 come into close contact with the inner circumferential surface of the tube 100 when the shaft 110 is slid into the tube 100 in a state in which torsional torque and/or rotational torque are applied to the shaft 100, so that sliding frictional force is increased between the solid parts 220 and the tube 100. With this construction, sufficient torsional fracture strength can be secured for the slip bush 200.
The conventional solid parts 220 are integrally formed with the slip bush 200. When the shaft 110 is slid in a state in which rotational torque is applied to the shaft 110, the solid parts 220 are maintained in a state in which the entire outer circumferential surfaces of the solid parts 220 are in contact with the inner circumferential surface of the tube 100. As a result, there is a problem in that the sliding frictional force between the slip bush 200 and tube 100 increases and a safety factor in relation to the torsional fracture strength decreases.
That is, it is necessary that when an external impact, such as a vehicle collision, occurs, the universal joint should collapse with the aid of the sliding movement between the tube 100 and the shaft 110, thereby absorbing the impact, and at the same time, reducing the length of the steering column 150 so as to protect a driver. However, when the above-mentioned conventional slip bush 200 is employed, there is a problem in that when an impact with torsion is applied to the universal joint, it is impossible to secure driver's safety because the universal joint does not properly collapse due to large frictional force between the solid parts 220 of the slip bush 200 and the inner circumferential surface of the tube 100.
In addition, with the conventional slip bush, the load in the process of transmitting rotational force between the shaft and the tube is mainly applied to the solid parts of the slip bush. If so, due to the property of the material of the slip bush which is equal to that of the solid parts, the solid parts may be fractured when strong rotational force is applied within a short period of time, and when such a phenomenon occurs, it is impossible to perform steering, thereby causing a serious problem in safety. This problem is caused when the above-mentioned safety factor in relation to the torsional fracture strength decreases.